This invention relates to a hollow fiber membrane (or "fiber" for brevity) formed on a tubular macroporous support. A thin tubular film of polymer on the cylindrical surface of the support, by itself, is non-self-supporting. The term "membrane" is used to refer to the hollow fiber membrane in its entirety, that is, the tubular film with the tubular macroporous support. A particular example of such a membrane is a tube of knitted or woven braid coated with the tubular film. In the art, a tube of braid having a nominal inside diameter of less than 2.5 mm, coated with a semipermeable film of polymer is referred to as a "hollow fiber membrane" or fiber. For the sake of clarity, reference to the film, by itself, is made with the term "film membrane", or "thin film" or "film" for brevity, since without the film there would be no membrane. Since the braid has macropores which are very large relative to pores within the film, they are referred to herein as "voids".
The non-supporting thin film is formed superficially on a tubular braid when a "dope" of a synthetic resinous material is coated on the outer circumferential surface of the braid without embedding the braid in the film. By "non-self-supporting" is meant that, even a short length of the tubular film, no more than about 10 cm long, extruded or otherwise formed with a circular cross-section, cannot support its own weight yet retain its circular cross-section. A tube having an inside diameter (i.d.) of 2.5 mm may be formed by either spinning a viscous solution of a polymer in an appropriate solvent ("dope") through a spinning nozzle having a circular rounding orifice of the appropriate size and passing a fluid through the axial bore of the nozzle to produce the bore of the tube; or, by forming the tube on a mandrel then collapsing the mandrel. However formed, because the tube of non-supporting film has such a thin wall, in the range from 0.01 mm to 0.09 mm thick, the tube will collapse unless supported by fluid. If a thin sheet of film 0.09 mm thick is either extruded or cast, a piece of the film in a small square 10 cm on each side, has so little strength that, by itself, it cannot be manually or mechanically manipulated without being damaged. Yet, and perhaps, more because of, its very thin cross-section and non-self-supporting nature, such a film, derived from the synthetic resinous material provides a semipermeable film having excellent semipermeability properties so long as the film is suitably deployed, and, a geometry favored by the film, is maintained.
If suitably adapted to satisfy the necessary criteria, the thin film will separate alcohol from a broth containing finely dispersed solids and live cells of microorganisms in the range from about 1.mu. to 44.mu. or larger in a microfiltration (MF) module, as described in detail in U.S. Pat. No. 5,250,182 to Bento et al; or, purified water from brackish water in a combination of ultrafiltration (UF) and reverse osmosis (RO) operations to produce potable water, as described in detail in U.S. Pat. No. 5,244,579 to Horner et al.
The problem was to find the conditions under which the film could be deployed. The physical solution lay in finding how to support the thin semipermeable film, less than 0.2 mm thick, preferably less than 0.1 mm thick, to provide an effective hollow fiber membrane in a practical application such as in the MF and UF of a fermentation broth to produce a permeate of aqueous alcohol and a concentrate of broth containing the sugar-containing components thereof, along with the microorganisms used in the fermentation; or, in the nanofiltration (NF) and reverse osmosis ("RO") of brackish water.
Semipermeable composite membranes are old in the art, and are also referred to as "reinforced semipermeable membranes". Such reinforced membranes are referred to in U.S. Pat. No. 4,061,821 to Hayano et al (the '821 patent, for brevity); in U.S. Pat. No. 3,850,203 to Shobert (the '203 patent, for brevity); and, in U.S. Pat. No. 3,644,139 to Schwarz (the '139 patent). In each of the references is taught a self-supporting film which is further reinforced with a fibrous material.
Specifically the '821 patent teaches both a sheet or flat membrane as well as a hollow fiber membrane of polyacrylonitrile, or, a copolymer of acrylonitrile and methyl acrylate which by itself was used by Hashino et al in U.S. Pat. No. 3,871,950 because of its "much greater water-permeability than the conventional products, a large mechanical strength, few clogging (sic), and capability of performing continuous filtration operation for a long period of time with the same material, a high chemical stability and a superior resistance to microorganisms." (see col 2, lines 13-17). Copolymers had been used by Schwarz in the '139 patent, to coat both sides of a cellulosic sheet of paper thus stabilizing the polymer with the cellulosic sheet.
But Hayano et al found that with any semipermeable membrane, there is a "restriction in the actual use because the water permeability is extremely reduced and/or the shape of membrane cannot be maintained when they are brought into contact with hot water or they are dried." (see col 2, lines 50-54). With particular regard to hollow fiber membranes such as were disclosed in U.S. Pat. No. 3,674,628 to Fabre, and those of Hayano et al and Hashino, though suffering from the stated limitation, were, for their purpose, highly satisfactory semipermeable membranes.
Thus, the '821 reference provided "a fabric as reinforcing material in case of flat type, and a braid having a central hollow portion as reinforcing material in case of hollow fiber type." (see col 2, lines 6-9). In each case, the flat fabric or tubular braid reinforcement provided a network of openings referred to herein as "voids" in the reinforcement which was embedded in the membrane so as to fix the polymer membrane within the openings. By using the fabric or braid for reinforcing the membrane against rupture and inadvertent damage, the mechanical strength and stability under pressure of the hollow fiber membranes was enhanced by the fibrous reinforcing material.
When a thick tubular membrane was formed, superficially coating the surface of the tubular braid, the stabilizing effect of the openings in the reinforcing material was lost, as stated by Hayano et al, in the sentence beginning at the bottom of col 4, and bridging cols 4 and 5 (near the top), and their reinforced membrane was not an effective membrane.
In view of the specific teaching of Hayano et al that a tubular braid only superficially coated with a membrane film (film membrane) is not a desirable embodiment for a semipermeable membrane, it was particularly unexpected to find that, within the limits stated herein, the semipermeable membrane of this invention is a highly effective one.
It is recognized that flux through a membrane (and flow of permeate) is maximized, when the membrane is made as thin as possible. If such a membrane is to be used to produce a continuous flow of permeate under substantial pressure difference sufficient to provide continuous flow, it is the received wisdom in the art to reinforce such a thin membrane against rupture. This is typically accomplished with a reinforcing means in contact with the membrane, the reinforcing means perforce extending over a relatively large portion of the membrane if the transmembrane fluid pressure difference is relatively large. The presence of a reinforcing means over such a large area thus significantly reduces the effective bare area of the membrane in contact with fluid on the low pressure side of the membrane. It also undesirably reduces the area of the membrane from which permeate can flow unobstructedly away from the membrane; and, rinse fluid can flow unobstructedly against the membrane. Additionally, such a reinforcing means typically provides dead spots where fluid flow rate is reduced causing concentration polarization. Both phenomena reduce the effective flux through the membrane. Thus, using a tightly woven braid for reinforcement would certainly appear to be at cross purposes with using an asymmetric thin film membrane. Even without Hayano et al's specific teaching that overlying a braid with a membrane, irrespective of its thickness, or whether the membrane had a proclivity to shrink, it seemed fatuous to expect that their teaching missed the mark.
Based on the received wisdom, Caro et al in U.S. Pat. No. 4,787,982 placed their "flaccid" reinforcement on the outside of their asymmetric thin film membrane, not the inside. By "flaccid" is meant that the denier of monofilaments used in the yarns or "ends" for carriers which are braided, and the number of picks/unit length of the braid, are such that a tubular braid has very little mechanical strength in a vertical plane normal to its longitudinal central axis, so that is so flexible that it can be easily manually tied into a knot. A typical braid starts out as multiple filaments which make up a single "end" and two "ends" are plied together in 3.8 twists/25.4 mm to make up a yarn or "carrier". Multiple carriers, preferably 24, are used to braid a tubular braid.
From a different physical viewpoint, a tubular braid having an i.d. of about 2 mm, when viewed resting longitudinally on its cylindrical surface under a microscope, does not present a geometrically cylindrical cross-section. The upper portion of the tubular braid sags indicating the cross-section is asymmetrical and that the wall of the tubular braid, in cross-section has very little rigidity. As the diameter of the braid decreases, there is progressively less sag, but even a tubular braid having an inside diameter as small as 0.25 mm is so flexible that it will be depressed to the point of near-collapse, under light finger pressure, no more than about 0.25 lb-force. Yet, after the dope is coagulated on the braid to form the film, the hollow fiber membrane formed can withstand a hydraulic pressure high enough to permit its use as a RO membrane, typically up to at least 12,500 kPa (1800 psig).
As will presently be evident from the data presented, both Hayano et al and Caro et al missed discovering the essential physical facts.
The function of the braid in Hayano et al is clearly that of a reinforcing support which tends to negate the shrinkage otherwise known to occur with such a membrane film; and, the function of the supporting braid is to stabilize the network of pores in a polymer film by-embedding the braid in the polymer which is peculiarly susceptible to the shrinkage problem. However, a thick-walled film for a film membrane in which is the braid is embedded, reduces available membrane surface area and the thick membrane wall is directly responsible for reduced permeation. Since Hayano et al were mainly interested in a pervaporation membrane which was to operate at elevated temperature where the pores of an acrylonitrile-containing film shrunk, they were willing to sacrifice flux for the ability to use a material which was inert to the fluids in which it was to be used. Under the circumstances, it did not matter to Hayano et al whether the polymer is coagulated from the inside of the braid or the outside, as long as the pores of the membrane were kept open, and this was accomplished when the braid is embedded in the film formed. The function of the braid in Hayano et al was to provide a stable network of reinforcing carriers which negated shrinkage of the pores in the film. To make our film supported on the outer surface of a tubular braid having an essentially circular cross-section, referred to herein as a "braided membrane", the dope can only be coated onto the outer surface of the braid, and the dope-coated braid is contacted only from the outside with a coagulant. The bore of the braid remains uncoated.
Neither was there any teaching in Hayano et al as to which materials in filament form could be spun into "ends" which would provide the necessary adhesion with any particular generic class of polymers. In general, membranes formed by embedding a braid in a polymer, upon being used, result in the polymer becoming detached from the braided carriers ("peeling"). For example, a polysulfone film peels off a braid of Kevlar aromatic polyamide fibers. Most significantly, the prior art recognized that polymers which contained a repeating unit derived from acrylonitrile provided a useful pervaporation membrane at about 80.degree. C. when the braid was embedded in the membrane, but failed to realize that such polymers were unusable as/ton-self-supporting semipermeable membranes when supported superficially on a tubular support having interstitial voids.
The thin film used herein, by itself, cannot be used as a semipermeable membrane in any practical sense. Further, except in the instance when a substantially rigid braid is used (for example one woven from relatively stiff carbon or graphite fibers, as opposed to a highly flexible braid woven from very flexible carbon fibers), the tubular braid typically used is flaccid.
With so little mechanical strength in a vertical plane normal to its longitudinal central axis, it appeared unlikely that such a braid might provide a tubular platform upon which to cast a membrane and afford a desirable braided hollow fiber membrane. There was no logical reason to expect that, supported by the tubular platform, the braided hollow fiber membrane may be operated for MF or for UF under a vacuum drawn on the "lumens" (bores of the fibers) in the range from 1 mm to about 100 mm of Hg, and under an overall differential in hydrostatic pressure in the range from about 110 kPa to 300 kPa for MF flow, from about 300 kPa to about 690 kPa for UF flow, and from 690 kPa to about 7000 kPa for NF or RO flow, the highest differentials being for RO flow.
A plausible explanation of what occurs when the tubular braid is coated with a thin film of the semipermeable membrane, without embedding the braid in the membrane, is that the forces generated within the polymer film, as the film is formed from solution, tend to distend the tubular braid and maintain its circular cross-section under tension. As solvent for the polymer is removed the tensile forces exerted are high enough to distend the braid into an essentially right cylindrical shape, and this circular cross-section of the coated braid is maintained even under high pressure differential, during operation.